I've done this a few times, usually successfully, but for some reason it doesn't always seem to work.
I edited this one today & as far as I can see, only the first image has successfully linked to a 'medium' image; the other still links to 'large'.
Can we tell if this is an error on my part, an imgur issue, or if they have implemented some new structure?
I've been playing with this one troublesome image a while to see if I can see what's going on; even so far as to re-upload it to see if anything changes [it doesn't].
s and b sizes work on it, though, just not m.